Power supplies are filtered to prevent noise from being transferred from the power supply to the load. Noise may emanate from various sources, including ripple from an alternating current (AC) power source that is rectified to provide direct current (DC) voltage from the power supply. Furthermore, loads that receive energy from the power supplies may emanate noise that returns to the power supply and is potentially distributed to additional loads being powered.
To prevent noise from being distributed by a power supply to its associated loads, a power supply filter is used to suppress the noise. The power supply filter generally contains capacitive and/or inductive components that obstruct frequencies that contribute to the noise. To provide a steady level of DC voltage, large capacitors may be connected in parallel with the power supply to suppress any voltage ripple or other noise. When the power supply is first energized, the large capacitor may contain little or no charge and as a result a large in-rush current may occur to instantaneously charge the capacitor.
For safety, power supplies employ over-current protection such as a fuse or circuit breaker. The large in-rush current that may occur may be much greater than the normal operating current being drawn by the load of the power supply. Therefore, the large inrush current blows the fuse or trips the circuit breaker, and proper operation of the power supply is inhibited.
Attempts to prevent the in-rush current from aggravating the over-current protection have included operators manually connecting a resistor in series between the charge capacitor and power supply for a period of time and then removing the resistor to directly reconnect the capacitor across the power supply output. For relatively high-voltage power supplies, the charged capacitor presents a dangerous potential for electrical shock when the operator is manually handling leads from the capacitor.
Thus, power supply filters are a necessary feature of power supplies, but they result in additional problems and hazards due to the in-rush current problem.